Transmit systems generally include a set of phase-locked loops. Each phase-locked loop includes a voltage-controlled oscillator. The principle of such transmit systems is to mix waves produced by the phase-locked loops to produce a transmit wave. There is generally a first loop whose oscillator oscillates at an intermediate frequency, for example around 100 MHz or 200 MHz, and a second loop whose oscillator oscillates at a translation frequency. In the Global System for Mobile Communications (GSM), for example, the translation frequency can be in the order of 800 MHz for transmission at around 900 MHz. In the Digital Cellular System (DCS), it is in the order of 1,700 MHz for transmission at around 1,800 MHz. The waves at the intermediate and translation frequencies are usually combined directly. They can equally be combined in a third loop. The signal from the intermediate frequency loop then serves as a set point for a phase comparator of the third loop, which receives a signal resulting from subtracting the signal at the translation frequency from a signal at a transmit frequency.
A phase-locked loop essentially comprises a voltage-controlled variable frequency oscillator whose output is connected to the input of a frequency divider. The output of the frequency divider is connected to the input of a phase comparator which also receives a set point signal at a frequency to be complied with. The phase comparator delivers a control signal which is preferably filtered by a low-pass filter. The control signal delivered by the low-pass filter controls the voltage-controlled oscillator. At the output of the oscillator, this produces a signal whose frequency is the frequency of the signal injected as a set point into the phase comparator multiplied by a divider coefficient of the frequency divider.
The frequency hops, corresponding to multiples of a standard channel bandwidth, are multiples of standard values. The GSM uses channel bandwidths of 200 kHz, for example. The various channels are therefore spaced from each other by a frequency increment equal to 200 kHz.
In the prior art the divider coefficient of the divider incorporated in the frequency translation loop is modified to bring about the required frequency hops by addition/subtraction. In the GSM, for example, with an increment of 200 kHz and translation frequencies equal to 1,700 MHz, the divider coefficient is in the order of 8,500. The coefficient varies by one or more units according to whether the required frequency hop is one or more times the 200 kHz increment.
A solution of the above kind is not totally satisfactory because the translation frequency loop with a frequency divider unfortunately generates unwanted noise in the transmitted band, despite all the precautions taken in implementing it. The oscillator and all components in a loop of the above kind produce noise. The noise at the output of a transmit system of the above kind is essentially proportional to the divider coefficient of the frequency translation loop divider.